Many modern engines utilize selective catalytic reduction (SCR) catalysts to reduce NOx emissions from the system. An SCR catalyst is a dynamic component that adsorbs reductant (e.g. NH3) and NOx from the exhaust gas, and reacts the reductant with the NOx to reduce the NOx. The amount of NH3 storage depends upon the temperature of the catalyst, and the NOx conversion capacity of the catalyst depends upon the temperature, the amount of NH3 stored on the catalyst, and/or the flow rate of NOx and exhaust gases through the catalyst. Accordingly, under certain operating conditions, transient events with respect to temperature, exhaust flow rate, and/or NOx composition of the exhaust gases can cause undesirable operations of the SCR catalyst.